Spark plug



June 12, 1934.

J. w. PARKIN, JR

SPARK PLUG Filed Aug. 12, 1931 2 Sheets-Sheet l 3mm .IZParfLin,JE?

J. W. PARKIN, JR

SPARK PLUG Filed Aug. 12, 1931 2 Sheets-Sheet 2 .4% a Wra h ,1121

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June 12; 1934.

Z m 37% 3 w r Patented june 12, 1934 SPARK PLUG Joseph W. Parkin, Jr.,Philadelphia, Pa. Application August 12, 1931, Serial No. 556,648

3 Claims.

This invention relates to spark plugs and proposes improvements in theconstruction and arrangement of the cooperating elements at the sparkingend of the plug by which the spark is rendered fatter and more intense,the spark plug self-cleaning, and the optimum temperature valuemaintained of the insulator and its appurtenant parts.

One of the objects of the invention is to construct a spark plug havingan annular gap between the middle electrode and shell, with the end faceof the insulator interposed in the path of spark travel across said gap,whereby the insulator is kept clean by the detergent action of the sparkitself.

Another object of the invention is the provision of means for increasingthe depth of the ionized path which the spark traverses, so as to spreadthe spark, presenting a greater surface to contact with the combustiblemixture.

A further object of the invention is'to provide a metallic shield forthe insulator, projecting beyond the threaded zone of the shell andsurrounding the insulator, absorbing heat at the peak of the explosionand giving it back to the insulator by radiation as the temperaturecurve of the exploded mixture declines so that the temperature of theinsulator is maintained at a substantially even high value at which thehydrocarbon will not condense upon the surface of the insulator.

Still another object of the invention is to construct a spark plug whichnormally sparks to a wire ground electrode, but as soon as the end ofthe insulator starts to foul, transfer the sparking path to the surfaceof the insulator, functioning thereafter in the manner previouslydescribed until the dielectric value of the insula tor surface has beenrestored, whereupon the sparking circuit shifts back to its normal pathbetween the central electrode and the wire ground electrode.

Other objects of the invention will appear as the following descriptionof a preferred and practical embodiment thereof proceeds.

In the drawings in which the same characters of reference have been usedthroughout the several figures to designate identical parts:

Figure 1 is a perspective view of a spark plug embodying the features ofthe present invention;

Figure 2 is a plan view of the inner end of the spark plug;

Figure 3 is a longitudinal section through the inner end;

ionized path of spark travel and the detergent action of the spark uponthe peripheral lip surrounding the lower end of the insulator;

Fig. 5 is a similar view showing a contrasting construction in which theinsulator projects to fall into the combustion chamber causing the sparkto are over the lip of the insulator without contacting with the same;

Fig. 6 is a cross section taken along the line 66 of Figure 3;

Fig. 7 is a modified form of the invention;

Fig. 8 represents still another modification, the insulator being formedof mica; and

Fig. 9 is a plan view of the plug shown in Figure 8.

Referring now in detail to the several figures, the numeral 1 representsin general, a spark plug having the insulator 2, the shell 3 and thecentral electrode 4 mounted in the insulator. Figure 3 shows that theinsulator seats in the shell in the usual manner by means of coopcratingshoulders between which a gasket 5 is compressed. The shell has apreferably frustoconical shield 7 projecting beyond the threaded zone ofthe shell and the skirt 8 of the insulator is sufficiently long as tobring the end of the insulator just a trifle beyond the peripheral edgeof the shield 7. A narrow inert gas space 9 is formed between the skirtof the insulator and the shell.

The insulator is preferably slightly cupped at the end as is indicatedat 10 in Figure 3 forming a peripheral lip 11 and the central electrodeprojects slightly beyond the bottom of the cupped portion as shown at 12in Figure 4. Y

An annular gap is formed oetween' the central electrode 10 and theperipheral edge of the shield '7. The end surface of the insulatorobstructs a portion of this gap and constitutes a path for the passageof the spark which crawls along the surface of the insulator and thenleaps the narrow gas chasm 13 remaining between the insulator and theshell. When the spark plug is new and clean, there is no preferred pathfor the spark and the current therefore, generally divided into aplurality of sparks leaping radially in several directions across thesurface of the insulator and across the gas chasm 13 to the shell.

If the plug becomes fouled at any point on the end of the insulator, theresistance of the gap is reduced at this point and it becomes thepreferred path of spark passage. The spark passing at this point quicklyburns up the fouling deposit and the spark plug is again clean. Theprovision of the gas chasm 13 is principally to prevent a short circuitin the event that the surface of the insulator becomes fouled. If itwere not for this gas chasm, the current would leak across the fouledarea of the insulator without producing a disruptive discharge.

The spark during the greater part of its passage across the gap and bythe term gap, it will be noted that the end surface of the insulator isregarded as a part thereof, the spark crawls or contacts with thesurface of the insulator. This of course presents only the outer surfaceof the spark to contact with the explosive mixture and some uncertaintyof the explosion might result. It is highly desirable to spread thespark depthwise so as to bring a greater ignition surface into contactwith the explosive mixture. This is accomplished by increasing the depthdimension of the path of ionization which the spark traverses in itstravel across the gap. With the central electrode terminating flush withthe surface of the insulator in the manner indicated in Figure 5, thespark will be flat and only the outer surface thereof will be in contactwith the combustible mixture. In Figure 4, the central electrodeprojects a very slight distance beyond the bottom of the cup portion 12and so produces an ionized path of considerable depth as is indicated bythe dotted lines in Figure 4. Experiments have shown that the centralelectrode extends a considerable distance beyond the cupped portion 12,the path of ionization will not be correspondingly deepened but willemanate very close to the base of the projecting part of the elec trodein the manner indicated by the dotted lines. Wnen the spark traversesthis deepened sides are rendered available for ignition contact with thecombustible mixture.

While the maintenance of an insulator clean in its entirety is adesideratum in all spark plugs, it suffices if the peripheral lip of theinsulator presents a clean non-conductive surface. In the present sparkplug the insulator projects to a very slight degree beyond the end ofthe shell so that the course of the spark in changing its direction asit leaves the cupped portion of the insulator brings it into intimatecontact with the insulator at the lip and causes it to perform thenecessary detergent action in clearing the lip of all foulness. Theextent to which the insulator may project may be decidedly overdone andFigure 5 shows an instance in which the insulator projects to fallthrough the gap. The spark has a certain inertia and in traversing thesteep sides 14 of the cupped portion of the insulator, it flies beyondthe lip of the insulator before changing its direction and descendingtoward the shell. By so doing, it altogether misses making contact withthe lip of the insulator and in consequence, has no detergent actionupon said lip.

Figures 1 and 3 show that only a very small part of the insulator isexposed direct to the heat of the combustion chamber. While the skirt ofthe insulator projects Well inwardly beyond the threaded zone of theshell, yet the latter is provided with an inwardly extending andpreferably frustro-conical shield '7 likewise extending inwardly beyondthe threaded zone of the shell and surrounding with a fair degree ofcloseness the lower portion of the skirt of the insulator. At the momentof the explosion when the heat of combustion is at its maximum,

the insulator is protected from the intense heat, the heat being in themain absorbed by the shield 7. As the piston of the engine descends onits working stroke and the temperature curve of the expanding mixturequickly falls due to its doing work, the heat previously absorbed by theshield 7 is now given out by radiation to the insulator. The latter inconsequence, is not so hot as the combustible mixture at the peak ofheat of the latter, nor so, cool as the combustible mixture during themajor portion of its working stroke. In other words, the temperature ofthe insulator remains substantially normal and its value is such at alltimes that liquid combustible will not condense upon it, nor will theproducts of the explosion.

Figure 7 shows a form of the invention in which the spark normally leapsthe gap between the central electrode 4 and a wire ground electrode 14.The relation of the end of the insulator to the shell and middleelectrode is the same in this instance as in the previously describedforms of the invention, the gap between the electrodes 4 and 14 beingless than the gap value between the central electrode and the shell 15when the end of the electrode is clean. Should sufficient foulnessaccumulate on the end of the insulator to reduce the gap value acrossthe insulator in less time than that between the electrodes 4 and 14,the spark path will shift, the spark travelling from the centralelectrode over the end of the insulator along the path of leastresistance, leaping the chasm 13 to the shell 15. The disruptivedischarge along the surface of the insulator exerts both a mechanicaland burning effect upon the foulness restoring the insulator to cleancondition, whereupon, since the di-electric value of the spark pathacross the insulator has again exceeded that between the electrodes 4and 14, the spark path will shift back to the gap between the last namedelectrodes.

Figure 8 shows a form of the invention in which the insulator is builtup in known manner of a roll of mica surrounding the central electrodeand the pile of mica disks 16 surrounding the roll and compressedtogether. The mica disks are turned or otherwise shaped so as to givethe proper form to the insulator. In the case of mica it is impossibleto form a well in the end of the insulator such as that shown in Figure4. The central electrode is provided with an enlarged head 17 abuttingthe fiat lower end of the insulator, said end projecting slightly beyondthe end of the shell extension 7 so that the peripheral edge of the endof the insulator will be in the path of spark passage from the centralelectrode to the shell extension. The thickness of the head 17 causesthe ionized path of spark passage to be spread depthwise, thus producinga fat spark.

With the construction of the spark plug according to the presentinvention, it is practicable to build it of very small size. This has adistinct advantage, particularly in the engines of air-craft. A largepercentage of accidents in the operation of air-craft occurs during thetaking-off period. This is due to the fact that the spark plug is cold,that the combustible mixture condenses upon it and causes the engine tomis-fire and possibly fail before the aircraft has gained altitude. Bymaking the plug of very small size as may be done in the presentinvention, it heats up much quicker than does the ordinary spark plug sothat the engine attains an operating temperature at which hydro-carbonmoisture will not condense upon the insulator, much more quickly thanwhen using the ordinary spark plug.

While I have in the above invention disclosed what I believe to be apractical and preferred embodiment of the same, it is to be understoodthat such changes may be made in the construction of parts as to adaptthem to the varying exigencies of use and that the illustrativeembodiment is to be considered as exemplary but not as necessarilyrestricting the claims to the construction as shown.

What I claim is:

1. Spark plug including an annular shell, an annular insulator arrangedcoaxially within said shell and extending slightly beyond the plane ofthe end of said shell, and having a central well, a central electrodecarried by said insulator and projecting into and terminating withinsaid well, the spark gap thus formed between said electrodes comprisingin serial relation an annular peak of insulation surface, separated fromthe ground electrode by a deep annular chasm, and extending outwardlyslightly beyond the ground electrode, the wall of said well forming aspark path from said peak to the central electrode.

2. Spark plug as claimed in claim 1, the peak of insulating materialintersecting the otherwise direct path between said electrodes,determining a deflection in the direction of'the'passage of the spark.

3. Spark plug as claimed in claim 1, the extending portion of thecentral electrode determining a spreading 'of the spark within said welllongitudinally of said electrode.

JOSEPH W. PARKIN, JR.

